1. Field of the Invention
The present invention is related to the field of electrical technology/electronics and relates to a surface acoustic wave component that is composed of fan-shaped interdigital transducers. The invention can advantageously be used in filters and delay lines on the basis of surface acoustic waves, particularly in those filters and delay lines that have low insertion loss and great bandwidth.
2. The Prior Art
Surface acoustic wave components are known in which interdigital transducers are disposed on a piezoelectric substrate, the prongs and gaps of which transducers form groups having the same length, called cells, between them, and narrow in the average prong direction. The cells are structured like cells of unidirectional transducers, and at least one of the fan-shaped interdigital transducers is composed of two partial transducers.
As a result of the narrowing, prongs and gaps of the fan-shaped interdigital transducers become smaller, to the same degree, from one lateral delimitation to the other lateral delimitation, so that with an increasing frequency, the zone with significant wave excitation and significant reception migrates through the transducer being considered, in the averaged prong direction. The relative bandwidth of the transducers involved and therefore of the component is adjustable, depending on the size of the ratio of maximal and minimal prong width of one and the same prong.
A special embodiment is known from B. Steiner, “Optimising slanted-finger interdigital transducers (SFIT) filters” in Proc. 2001 IEEE Ultrasonics Symposium, pages 1-5, referred to as [1], in which unidirectional transducer cells of the DART type (Distributed Acoustic Reflection Transducer) are used in fan-shaped interdigital transducers, as narrowing cells. This makes it possible to combine the advantages of the possible broad bandwidth of fan-shaped interdigital transducers and the low losses of SPUDT (Single Phase Unidirectional Transducer).
The disadvantage of this special transducer is that it excites not only the basic frequency (1st harmonic), but also signals having double the frequency of the basic frequency, and also receives them. These signals are referred to as 2nd harmonic, whereby the ordinal number standing in front of “harmonic” will be referred to as the order of the harmonic hereinafter. In the case of filters that contain this type of fan-shaped interdigital transducer, the 2nd harmonic can significantly reduce the stopband attenuation, particularly if these filters have a very broad band. The same holds true for fan-shaped interdigital transducers having cells of the Hanma-Hunsinger type. Instead of the 2nd harmonic, the 3rd harmonic can lead to problems in the stopband range in filters that contain this transducer type.
Frequently, fan-shaped interdigital transducers are structured with prongs of equal width and gaps of equal width (H. Yatsuda, “Design techniques for SAW filters using slanted finger interdigital transducers,” IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control Vol. 44 (1997), pages 453-459), referred to as [2], in which transducers the polarity changes from prong to prong. The disadvantage of these interdigital transducers, structured in fan shape, lies in the fact that the advantage of reducing losses by means of the use of unidirectional transducer cells no longer comes to bear.
A further special embodiment of a surface acoustic wave component that contains interdigital transducers having prongs of equal width and gaps of equal width is described in U.S. Pat. No. 6,856,214 B2, referred to as [3]. The component consists of two filters that each consist of two fan-shaped interdigital transducers having prongs of equal width and gaps of equal width. Each fan-shaped interdigital transducer of the first filter is switched in parallel with a fan-shaped interdigital transducer of the second filter. As a result of narrowing of their transducer structures, the filters can be considered in such a way that they contain different frequency tracks along the aperture of the transducer. These frequency tracks are characterized by the period length λj, in each instance, where j=1, . . . m, whereby m is the number of frequency tracks, and the center frequency of the frequency track is determined with the index j of λj. The fan-shaped interdigital transducers of each filter enclose an interstice. The difference of the transducer interstices of the two filters in the frequency track with the index j amounts to λj/4 or an odd-numbered multiple of this. As a result, the waves transmitted by the transducers operated as transmitters and the waves reflected at the transducers operated as receivers compensate one another when they impact the transmitter. As a result, the triple-transit signal, which causes a ripple in the passband of the component, is reduced.
A further special embodiment of a surface acoustic wave component that contains fan-shaped interdigital transducers is known from DE 199 25 800 C1, referred to as [4]. The excitation and reception strength of some cells is reduced by serial switching of sections of these cells that are formed by division in the transversal direction. In this way, a specific weighting of the fan-shaped interdigital transducers can be set, and as a result, the transmission behavior of the filters in question can be improved.
In the embodiments [1] to [4], all the extended prong edges of a fan-shaped interdigital transducer intersect at a point, called the focus, so that each fan-shaped interdigital transducer has a separate focus.
A further known embodiment relates to a surface acoustic wave component that contains step-shaped fan-shaped interdigital transducers. Aside from the step-shaped character of the prongs, the extended prongs of the two fan-shaped interdigital transducers run toward a common focus (DE 199 43 072 B4). In the case of this type of transducer, as well, there is the disadvantage that the harmonics of the 2nd or 3rd order are not suppressed or are only slightly reduced.